1) Field of the Invention
The present invention relates to an image processing apparatus that compresses an input image signal to be stored in a memory, reads out the stored image signal at a predetermined timing, decompresses the signal, and outputs the signal to a recording unit.
2) Description of the Related Art
An image processing apparatus such as a digital color copier obtains a color image by superposing plates of four color as black (K), cyan (C), magenta (G), and yellow (Y) on one another and printing the plates. In this case, images need to be stored temporarily in a memory in order to control timings of printing the color plates, which causes a required memory capacity to be enormous. To prevent the increase in the memory capacity, a method of compressing image data and storing the data in the memory, has been employed. Further, users have requests to not only output a captured image but also store the image as digital data and even use the data on a personal computer. In this case also, the image data needs to be compressed to an appropriate data amount. In either of the cases, compressing data at a certain stage during image processing is getting common in digital color copiers.
Furthermore, to improve image quality, processes as follows are carried out. That is, an image area is separated from each pixel area in data for a captured image, and it is determined whether the image area is a character area or a halftone area such as a photographic image. Area data indicating if the image area is the character area or the halftone area is generated for each pixel, and the area is subjected to image processing according to the area data. More specifically, the processing is switched between processes of filtering and halftoning according to the pixel area.
A conventional image processing apparatus is disclosed, for example, in Japanese Patent Application Laid Open No. HEI 9-186866. This image processing apparatus includes a first image area separating unit that separates a binary image area comprised of a character and a line from a halftone image area such as a photograph and a halftone dot print, in an input image signal, and generates an image signal. The image processing apparatus further includes a second image area separating unit that subjects the image signal to first image processing based on the separation data, compresses and stores the processed image signal, decompresses the stored image data to generate an image signal, and determines if there is an edge in the decompressed image signal. The second image area separating unit then determines one area as an area with an edge (“edge area”) and another area as an area without an edge (“non-edge area”), and subjects the data to second image processing based on the result of determination.
That is, in the image processing apparatus, the second image area separating unit determines the presence or absence of the edge in the compressed and decompressed image signal, and subjects the two image areas, the edge area and the non-edge area, to different types of processing (switching of the halftone processing between the numbers of lines such as a 200-line screen and a 400-line screen). It is, thereby, thought possible that the sharpness of a character portion is compatible with the tone and granularity of a halftone image portion such as a photograph and printed photograph. Furthermore, the image data before being compressed and decompressed is divided into the character area and the photograph area by the first image area separating unit, and both of the areas are subjected to the processing for an appropriate spatial frequency during the first image processing. In the second image area separating unit, high separation capability can be obtained because the areas have wider latitude for determination.
However, it is seen in many cases that an actual dot image (photographic print image) has an area with a large edge amount. Like the image processing apparatus, even if an enhancing filter is used for the character area and a smoothing filter is used for the dot image in the first image processing, the dot image with a screen having a small number of lines is often subjected to insufficient smoothing, and therefore the second image area separating unit may determine that the image has an edge. Further, the image processing apparatus binarily determines the presence or absence of an edge according to an edge determination threshold. Therefore, some dot-line images inevitably obtain a problem such that the image has an edge area and a non-edge area at the same time.
Therefore, if a dot image is subjected to strong smoothing filtering, setting may be possible so as to determine that a dot image with any number of lines has no edge. However, if such a strong smoothing filtering is subjected, the character on the dots is also subjected to the strong smoothing processing, and therefore the character becomes a blurred image with no sharpness therein, which is not practical. A smoothing filter or a band pass filter is generally used so as to leave the sharpness of characters to some extent. Alternatively, an adaptive edge filter is used so as to control the intensity of the filter based on an edge amount. As explained above, such filter characteristics result in coexistence such that the edge area and the non-edge area coexist in the dot image. Therefore, a reproduced image does not seem natural because the 200-line screen and the 400-line screen coexist in the same image.
Particularly, determination through the second image area separation as to whether the edge exists is performed on the image data after being subjected to irreversible compression and decompression. Therefore, the determination becomes unstable because of influence of image deformation due to the processing of compression and decompression. Thus, coexistence of the different determination results is made more significant in the dot image. Because of this, when adaptive image processing is selected based on the determination, as to whether the edge exists, that is performed on the image data having been irreversibly compressed and decompressed, a defect (poor and inferior image quality due to the coexistence) occurs in an area where the determination results coexist.